Substances which have relatively low NQR frequencies (perhaps 1 or 2 MHz or less) and relatively long values of T1 (perhaps 500 ms, 5 or 10 s or more) include the explosives PETN and TNT, Potassium Nitrate (KNO3) and 27Al in alumina. For example, PETN has resonance frequencies around 0.9 MHZ, a T1 of roughly 30 s at room temperature, as well as a spin-spin relaxation time (T2) of approximately 20 ms. It is noted in passing that T2 is preferably defined herein as the exponential constant measured by means of a Hahn echo or similar pulse sequence.
NQR testing is used for detecting the presence or disposition of specific substances, and in particular polycrystalline substances. It depends on the energy levels of quadrupolar nuclei, which have a spin quantum number I greater than xc2xd, of which 14N is an example (I=1). 14N nuclei are present in a wide range of substances, including animal tissue, bone, food stuffs, explosives and drugs. One particular use of the technique described herein is in the detection of the presence of substances such as explosives or narcotics. The detection may be of baggage at airports, or of explosives or drugs concealed on the person or buried underground or elsewhere. Other nuclei of interest are 27Al(I=5/2) and 63Cu(I=3/2). 27Al is present in minerals, cement and concrete, whilst 63Cu is present in ores and many high Tc superconducting materials.
In conventional Nuclear Quadrupole Resonance testing a sample is placed within or near to a radio-frequency (rf) coil and is irradiated with pulses or sequences of pulses of electro-magnetic radiation having a frequency which is at or very close to a resonance frequency of the quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiant energy will generate a precessing magnetization which can induce voltage signals in a coil surrounding the sample at the resonance frequency or frequencies and which can hence be detected as a free induction decay (f.i.d.) during a decay period after each pulse or as an echo after two or more pulses. These signals decay at a rate which depends on the time constants T2* for the f.i.d., T2 and T2e for the echo amplitude as a function of pulse separation, and T1 for the recovery of the original signal after the conclusion of the pulse or pulse sequence.
As described in International Patent Application No. WO 96/26453 in the name of British Technology Group Limited, the subject matter of which is incorporated herein by reference, spurious interfering signals (also termed xe2x80x9cringingxe2x80x9d) which are not associated directly with or due to the nuclear resonance may sometimes arise from a sample during NQR tests.
For example, one group of materials which can cause interference problems includes metallic conductors. Such materials may be commonly found in many types of objects in baggage. It has been discovered that the interference may be particularly pronounced when a sample includes metallic or ferromagnetic material as a layer of plating on another material, especially, it has been found, when the plating layer comprises Nickel. Objects which are particularly prone to such problems include screws or key-rings. The cause of this type of interference has not been proven, but it is believed to emanate from ferromagnetic or like resonance effects in the B1 field of the sample coil, and be due to a form of magneto-acoustic ringing. It should be emphasised that this interference is not an artefact of the particular detection apparatus used, but a feature of the material itself. Also it will be understood that, in the context of the detection of the presence of a particular substance in a sample, it would usually not be the particular nuclear species to be detected but the remainder of the sample which would give rise to the interfering signals.
The spurious interfering signals (or xe2x80x9cartefactsxe2x80x9d) commonly have decay characteristics very similar to those of true NQR signals, and, furthermore, are often many times stronger; they can last for several milliseconds. The phase of those inter fering signals and that of the resonance response signal following a single radio-frequency excitation pulse are entirely determined by the rf phase within the pulse. There is, however, one important distinction. When two or more pulses are used, the phase of the NQR response signal, whether it be a free induction decay (f.i.d.) or an echo, depends on the relative phases of the two preceding pulses, unlike that of the interfering signal, which is determined almost entirely by that of the immediately preceding pulse.
This distinction has been exploited in WO 96/26453 in an attempt to remove the interfering signal from an NQR response signal. The proposed solution involves the use of at least one pair of excitation pulse sequences (or blocks) in which the phase of the pulses is controlled in such a way that when the response signals from the two member sequences of the pair are compared the spurious signals can be largely eliminated whilst the genuine NQR signals can be retained.
It has been discovered pursuant to the present invention that, when applying a multiple pulse sequence such as one of those described in WO 96/26453, the response off-resonance varies with frequency in a periodic fashion. An example of a typical off-resonance response to a multiple pulse sequence for a typical substance is shown in FIG. 1. The response has been found to have narrow peaks and wide troughs which are believed to be due to the pulsed nature of the excitation; the separation of the peaks is believed to be related to the pulse repetition rate. Furthermore, the resonance frequency of the peaks varies with temperature and other such environmental parameters. Unless the excitation is exactly at the resonance frequency, or exactly at the frequency of one of the other peaks, there will not reliably be any response signal. Therefore, for example, in a typical situation (such as airport security checking) where the exact temperature of the sample is not known, the usefulness of such multiple pulse sequences may be reduced.
Further, it has been found pursuant to the present invention that the off-resonance behaviour of multiple pulse sequences can cause particular problems when they are used in pairs as described above to reduce spurious signals, particularly if the substance under test has a relatively low resonance frequency and/or long spin-lattice relaxation time.
The present invention seeks to maintain or improve upon the level of spurious signal suppression achieved using the technique described in WO 96/26453, but to improve the off-resonance response, especially for long T1 substances. The invention also seeks to improve the sensitivity of NQR tests. The invention is based in part upon the discovery, pursuant to the invention, that an improvement in the off-resonance response and the sensitivity of the NQR test may result if the delay time between the two excitation pulse blocks mentioned above is carefully controlled.
Prior to the present invention, it was considered that sufficient time must be left between the two excitation blocks to allow the NQR magnetization generated during the first block to recover. However, it has now been discovered pursuant to the present invention that, by having a delay between the two blocks which is insufficient for the magnetization to recover, the off-resonant response behaviour and the sensitivity of the NQR test may be considerably improved.
According to the present invention there is provided a method of Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei exhibiting a given value of spin-lattice relaxation time, T1, the method comprising:
applying two (or possibly more) excitation blocks to excite nuclear quadrupole resonance, there being a given delay time between the two blocks;
detecting resonance response signals; and
comparing the response signals from respective blocks;
wherein the delay time is less than the T1 value of the nuclei.
By having a delay between the two blocks which is less than the T1 value of the nuclei (that is, a delay which gives insufficient time for the magnetization to recover), the off-resonant response behaviour and the sensitivity of the NQR test may be considerably improved. This may allow improved detection of substances displaying weak NQR signals, in situations where the exact temperature of the sample is not known.
Each excitation block (or sub-block) may comprise one or more excitation pulses which generates an NQR response. Preferably, each excitation block comprises at least two, three, five or ten pulses, although it may comprise a multiplicity of pulses, say more than one hundred or even more than one thousand pulses. Suitably, the separation between each pulse may be less than, preferably less than one tenth of, the ring-down time (decay time) of the spurious interfering signals. Preferably, the separation between the pulses in a block is the same. Preferably the separation between the pulses is as defined in WO 96/26453 in relation to the SSFP and PSL pulse sequences. For example, the separation may be less than ten times, or five times, or three times or twice the value of the free induction decay time T2*. Indeed, the separation may be less than T2* or a half T2*.
Preferably (in any embodiment whatsoever), where there are a plurality of pulses in each block (or sub-block), there is no phase alternation between those pulses. As used herein, the term xe2x80x9cphase alternationxe2x80x9d connotes a variation of phase of more than 90xc2x0, preferably more than 135xc2x0, and more preferably of roughly 180xc2x0. Accordingly, xe2x80x9cno phase alternationxe2x80x9d implies a variation of phase certainly less than 180xc2x0, preferably less than or equal to 135xc2x0, and more preferably less than or equal to 90xc2x0.
Preferably, the comparison takes the form of a combination of the responses from the respective blocks such that the NQR signal is enhanced while any spurious signals are reduced. In one embodiment, the comparison takes the form of a subtraction of the responses from the respective blocks, possibly with some weighting being given to one of the blocks to account for differences in the signal levels generated by the blocks. In other embodiments involving blocks having two or more constituent sub-blocks, the responses from the sub-blocks of one block are combined with the responses from either corresponding, or indeed non-corresponding, sub-blocks of the other block, such that the overall NQR signal is enhanced.
Advantageously, the delay time is less than half, preferably less than a quarter, more preferably less than a tenth and even more preferably less than a hundredth of the T1 value. In short, it is preferable that the delay time is very much less than the spin-lattice relaxation time of the nuclei.
It is also preferred that the delay time is greater than the spin-spin relaxation time, T2, of the nuclei, and hence advantageously the delay time is greater than once, preferably greater than twice, more preferably greater three times and even more preferably greater than five times the T2 value. This can ensure effective relaxation of the magnetization in the x-y plane.
On the other hand, preferably, the delay time is less than ten times and more preferably less than five times the T2 value, since this can maintain the duration of the test within a reasonable limit.
For typical nuclei of interest, preferred ranges of the delay time are between 1 and 1000 ms, preferably between 5 and 500 ms, more preferably between 10 and 100 ms and even more preferably between 20 and 60 ms.
One important feature of the present invention alluded to above is the discovery pursuant to the invention of the nature of the off-resonance performance in NQR of multiple pulse sequences. In order to improve the performance, preferably the first and second blocks, and the delay time therebetween, are arranged such that, if the resonance frequency of the nuclei were varied over a given range, the first and second blocks would generate response signals whose variation with frequency over the given range would in combination be less than for the response signals from separately either the first or second block. By arranging the first and second blocks and the delay time therebetween thus, the periodic variation of the response signals with frequency can be to an extent mitigated. It is in particular preferred if the peaks in the frequency response characteristic of one excitation block are arranged to coincide generally with the troughs in the characteristic of the other block, and vice versa.
It has been discovered pursuant to the present invention that the off-resonance response and the sensitivity of the NQR test may be further improved by applying excitation between the two excitation blocks. Therefore the method may further comprise applying excitation between the two excitation blocks. Preferably, the excitation is in the form of one or more excitation pulses, such pulses being termed herein xe2x80x9cbridging pulsesxe2x80x9d. By the use of such excitation the behaviour of the second block can be adjusted so that the overall pulse sequence can produce the desired improved result. In particular, the excitation between the two blocks can be used to improve the combined off-resonance behaviour of the two blocks.
In one preferred embodiment, an excitation pulse (herein termed xe2x80x9crefocussing pulsexe2x80x9d) is applied, at a time substantially coincident with the last echo generated by the first block.
In another preferred embodiment an excitation pulse (herein termed xe2x80x9cwindmill pulsexe2x80x9d) is applied at a time adjacent the centre of the delay time between the two blocks.
In another preferred embodiment, excitation pulses are applied between the excitation blocks to provide saturation. Such pulses may be termed xe2x80x9csaturation pulsesxe2x80x9d.
Various preferred features of the excitation applied between the two blocks are as follows.
If one (or more) excitation pulse is applied between the two blocks, the (or each) pulse may have an effective flip angle of between 20xc2x0 and 160xc2x0, or 200xc2x0 and 340xc2x0, or 30xc2x0 and 60xc2x0, or 70xc2x0 and 110xc2x0, or 160xc2x0 and 200xc2x0. It is noted in passing that the term xe2x80x9ceffectivexe2x80x9d in relation to a 90xc2x0 flip angle is used to connote the NQR equivalent of a Nuclear Magnetic Resonance (NMR) 90xc2x0 flip angle; in fact all flip angles referred to herein are xe2x80x9ceffectivexe2x80x9d flip angles.
If a plurality of excitation pulses is applied between the blocks, the second such pulse may be of the same or different flip angle as the first.
Each excitation block may comprise a first excitation sub-block and a second excitation sub-block, the response to one of the first and second sub-blocks in one block being compared to the response to one of the first and second sub-blocks in the other block. This can afford the advantage that by dividing the blocks into sub-blocks the off-resonance performance of the entire sequence can be enhanced, especially if the sub-blocks in each main block are different. Preferably, the response to the other of the first and second sub-blocks in one block is compared to the response to the other of the first and second sub-blocks in the other block as well.
This important feature is provided independently. Accordingly, the invention provides a method of Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei exhibiting a given value of spin-lattice relaxation time, T1, the method comprising applying two excitation blocks to excite nuclear quadrupole resonance, each excitation block comprising a first excitation sub-block and a second excitation sub-block, there being a given delay time between the two blocks, detecting resonance response signals, and comparing the response to one of the first and second sub-blocks in one block and the response to one of the first and second sub-blocks in the other block, the delay time being less than five times the T1 value of the nuclei (that is, a delay which gives insufficient time for the magnetization to recover).
Preferably, the delay time is less than three times or twice the T1 value of the nuclei; and more preferably the delay time is less than the T1 value itself. Advantageously, the delay time is less than half, preferably less than a quarter, more preferably less than a tenth and even more preferably less than a hundredth of the T1 value.
The first sub-block in each excitation block may be different from the second sub-block in each excitation block. For example, each sub-block may comprise a plurality of pulses, and the repetition rate of the pulses in the first sub-block may be different from the repetition rate of the pulses in the second sub-block. This can make the off-resonance response different between the first and the second sub-blocks, so that the combined off-resonance response can be improved.
If each excitation block comprises a plurality of excitation pulses, preferably the time between the first and second such pulse in the first block is different from the corresponding time for the second block. This has been found to be a particularly effective way of improving the off-resonance performance of the combined response signal from the first and second blocks.
For efficiency and optimum reduction in spurious signals, preferably each excitation block comprises a multiplicity of excitation pulses and at least the majority of the pulses in one block are substantially the same as the corresponding pulses in the other block.
One multiple pulse sequence of particular efficiency has been found to be a Pulsed Spin Locking (PSL) type sequence. In putting such a sequence into practice with the present invention, preferably each block comprises an initial preparation pulse followed by at least one pulse of different phase from the preparation pulse.
The feature that the time between the first and second such pulse in the first block is different from the corresponding time for the second block can be used particularly effectively in the context of a PSL type sequence. Accordingly, preferably, for one of the blocks the time between the preparation pulse and the immediately following pulse is half the time between subsequent pulses in the block, whereas preferably for (the other) one of the blocks the time between the preparation pulse and the immediately following pulse is substantially the same as the time between subsequent pulses in the block.
Another particularly effective pulse sequence in the context of the present invention is a Steady State Free Precession (SSFP) type sequence. In putting this sequence into practice for the present invention, preferably each excitation block comprises a plurality of excitation pulses, the time between each such pulse being the same.
In fact, repeated use of an SSFP type pulse sequence in a T1 limited fashion has been foundxe2x80x94surprisinglyxe2x80x94to afford a number of benefits in reducing spurious interfering signals. Accordingly, the present invention provides a method of Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei exhibiting a given value of spin-lattice relaxation time, T1, the method comprising:
applying two (or more) excitation blocks to excite nuclear quadrupole resonance, there being a given delay time between the two blocks, each excitation block comprising a plurality of excitation pulses, the time between each such pulse being the same; and
detecting resonance response signals;
wherein the delay time is less than five times the T1 value of the nuclei.
Preferably, the delay time is less than three times or twice the T1 value of the nuclei; and more preferably the delay time is less than the T1 value itself. Advantageously, the delay time is less than half, preferably less than a quarter, more preferably less than a tenth and even more preferably less than a hundredth of the T1 value.
The method may further comprise comparing the response signals from the respective blocks.
One particular preferred embodiment has been found to be where the phase of each pulse is the same. Also, if a plurality of pulses is provided in each block (or sub-block), preferably each (or most) of the pulses in that block (or sub-block) has the same or nearly the same phase; this may exclude the initial pulse in each block, which may be of a different phase. Preferably each (or most) of the pulses in that block (or sub-block) have phases which are within 90xc2x0 of each other.
Each excitation block may comprise at least one excitation pulse, and at least one of the pulses may be a phase split pulse.
Although reference has been made above largely to the use of two excitation blocks, one or more further pairs of blocks with the appropriate delay (for example, less than the T1 value of the nuclei, as taught previously) between each block of the pair could be used. Each pair of blocks may have substantially the same delay between the blocks, or the delays may be different. Each pair may be applied at a (slightly) different excitation frequency, in order to improve off-resonance performance.
The invention also provides apparatus for Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei, comprising:
means (such as an rf probe) for applying two (or more) excitation blocks to excite nuclear quadrupole resonance, there being a given delay time between the two blocks:
means (such as the or another rf probe) for detecting resonance response signals from the blocks; and
means (such as a processor) for comparing the response signals from the respective blocks;
wherein the delay time is between 1 and 1000 ms, preferably between 5 and 500 ms, more preferably between 10 and 100 ms and even more preferably between 20 and 60 ms.
Preferably, the first and second blocks, and the delay time therebetween, are arranged such that if the resonance frequency of the nuclei were varied over a given range, the first and second blocks would generate response signals whose variation with frequency over the given range would in combination be less than for the response signals from separately either the first or second block.
Preferably, the excitation applying means is adapted to apply excitation between the two excitation blocks. The excitation applying means may be adapted to apply an excitation pulse at a time substantially coincident with the last echo generated by the first block. The excitation applying means may be adapted to apply an excitation pulse at a time adjacent the centre of the delay time between the two blocks. The excitation applying means may be adapted to apply an excitation pulse between the two blocks and the pulse may have an effective flip angle of between 20xc2x0 and 160xc2x0, or 200xc2x0 and 340xc2x0, or 30xc2x0 and 60xc2x0, or 70xc2x0 and 110xc2x0, or 160xc2x0 and 200xc2x0. The excitation applying means may be adapted to apply a plurality of excitation pulses between the two blocks, the second such pulse being of the same or different flip angle as the first.
Each excitation block may comprise a first excitation sub-block and a second-excitation sub-block, and the comparing means may be adapted to compare the response to one of the first and second sub-blocks in one block and the response to one of the first and second sub-blocks in the other block.
In a closely related apparatus aspect of the present invention there is provided apparatus for Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei exhibiting a given value of spin-lattice relaxation time, T1, comprising means for applying two excitation blocks to excite nuclear quadrupole resonance, each excitation block comprising a first excitation sub-block and a second excitation sub-block, there being a given delay time between the two blocks, means for detecting resonance response signals, and means for comparing the response to one of the first and second sub-blocks in one block and the response to one of the first and second sub-blocks in the other block, the delay time being less than five times T1, for example, between 1 and 1000 ms, preferably between 5 and 500 ms, more preferably between 10 and 100 ms and even more preferably between 20 and 60 ms.
The first sub-block in each excitation block may be different from the second sub-block in each excitation block. For example, each sub-block may comprise a plurality of pulses, and the repetition rate of the pulses in the first sub-block may be different from the repetition rate of the pulses in the second sub-block.
The present invention also provides apparatus for Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei, comprising:
means for applying two (or more) excitation blocks to excite nuclear quadrupole resonance, there being a given delay time between the two blocks, each excitation block comprising a plurality of excitation pulses, the time between each such pulse being the same; and
means for detecting resonance response signals;
the delay time being less than five times T1, for example, between 1 and 1000 ms, preferably between 5 and 500 ms, more preferably between 10 and 100 ms and even more preferably between 20 and 60 ms.
One particular preferred embodiment has been found to be where the phase of each pulse is the same.
Preferably, means for comparing the response signals from the respective blocks are provided.
Method and apparatus features of the invention may where appropriate be interchanged.